Process for preventing the formation of incrustation or scale in boilers caused by salts of silicic acid



Patented Jan. 24, 1928.

uuirsn STATES e a-r ant orri'cn,

JULIUS-OiSTERTAG, OF STUTTGART,

PROCESS FOR PREVENTING THE FORMATION OF INGBUSTATION 0R SCALE IN BOILERS CAUSED BY SALTS 0F EILIGIC ACID.

No Drawing. Appficaticn filed. .Tune 17, 1927, Serial No. 199,623, and in Germany November 26, 1925.v

Means for removing the silicic acid present in the Water used in boilers have hitherto beensought-for innvain; all the experiments made in this directionhaving-proved unsucce: ital. Suggestions have frequently been made, butthe practical application oi these su gestions did not give any successful result;

lt'hastrequently been proposed to render the silicic acid in a steam boiler harmless by adding suflicient soda to the feed Water to litv has been shown however by numerousenperiments that the concentration of silicic acid or of sodium silicate respectively in the boiler only needs'tobe verytslight in order to cause the formation of scale; This deviationfro n'the generally. applied theory has been obserycdin many cases which have been thoroughly lIlVEStIgEIt-Qd and as. a result of these observations a solution of this impor tant questi on was arrlvedati The present invention renders 1t possible to preventthe hitherto unavoidable deposit oil in crustation 1 or. scale .inlb'oilers, .Which, is

formed owing to the presence of salts of" silicic acid. The process employed for this purpose is so extremely simple that its appllcation does not give rise to any ClllfiCllllIleS Whatever, inasmuch'as it consistssimply in.

maintaining a definite concentration of. the

boiler Water asregards its'easily soluble salt content together with a corresponding degree of alkalinity of" the boiler Water by drawing-oilboiler Water and discharging it outside.

The following'exainple Will serve to explain the process.

Assuming the analysis of the untreated.

feed Water to be as follows:

' Mg, per litre.

BYIOO i CO, in the form of bi-carbonatewu;

then it can be calculated from these figures, since one degree ot hardness; German scale,

, million, that there will be:

A. total hardness of ll or 586-pa'rts per j or 143 parts equals .70 degree of hardness, U. S. scale,

parts per the ll. 53. degree denoting ten million.

A carbonate hardness of 10 per million.

A non-carbonate hardness of31 ark 143 parts per inillion. l

A challrhardness of 30 or 429pa1'ts p'er million.

A magnesia hardness of 10 or lliliparts per million.

ll this Water is purified by the addition" of soda which term is usedin this description to denote the carbonate, for the purpose hardness, then each meter otthe purified water will contain 506 grammes ot Na sOg,

20? grammes oil? NaCl and 21:8 grammesoi In. consequence of the continuous evapo ration this concentration. of salts will: in crease more and more sovthat on.tl1isac- (*onuiiu'm salt.

At the. present time it iscustomary, when using a sludge return process to keep' the degree 0t alkalinity of the Water inthe normal HCl for 5-0 ccm. ot the Wateni. e. so

thatl cubic metre of the Water in'the boiler contains 530 to a maximum 051060 grammesotexcess of soda calculated as Na; ,CO ,..ifor

the purpose of compensating for irregularities 1n the hardness of the Water, .in the measuring operations, 1n the evaporation,

in the composition of the soda andlso i orth,

and also especially tor the purpose of pr0- viding protection agalnstthe materialfolitheboiler being attacked. It now the Water in the boiler is titratedinlthe usualway with methyl orange and decinorma-l hydrochloric acid, large errors may arise inasmuch as the Na SiQ contained .in the said Water will be estimated as Na tlQ The test WllllllTQTB- fore leadone to believe that the Water con tains the prescril'ied amount of alkaliiiitfy due to soda, Whereas in reality, there may be only sodium silicate present; -Solong as soda is still present, those constituents capa bio of causing hardness. which still enter the boiler in'the iurified'uater' are reci oi oi precipitating the non-carbonate" tated in the form of a harmless sludge, which can be easily cleared out by the sludge return arrangements. Sulphate of calcium in particular is completely converted into calcium carbonate in the presence'ot an excess of soda in the water in the boiler, whereas when there is a deficiency oi" soda, the calcium sulphate at once settles on the walls of the boiler and forms an incrustw tion. It now there is no alkalinity due to soda in the waterin the boiler, but on the other hand silicate of soda is present, which has been estimated. in error as soda, and if constituents capable oi causing hardness enter the boiler with the purified water, then these latter will react with. the sodium silicate in similar manner to their manner of reactin with soda. The substances produced by this reaction, such as magnesium silicate and calcium silicate, do not, however, form a harmless sludge. but at once settle, exactly as calcium sulphate does when there is a deficiency of soda, on the walls of the boiler where they form an extremely hi 'hly insulating layer.

ihe problem which it is the object oi the present invention to solve is as follows:

It is intended that a true alkalinity due to soda shall he obtained in the boiler water, so that titration can be carried out in the ordinary way. In carrying this into effect it is intended that the error caused by the sodium silicate which is present shall be so small that it is no longer of importance, i. e. with a degree of alkalinity of. the boiler water equivalent to 510 ccm. of normal HCl for 50 ccm. of water titrated with methyl oran e, not more than 2 ccm. of the acid used shall go to neutralize Na SiO whilst the remainder neutralizes the Na CO Bearing in mind the analysis already. given the following calculations are arrived at:

1 com. of decinormal hydrochloric acid is 1 50 ccm. of water (i. e. the twenty-thousandth part of 1 cubic metre) are used for the test, 1 ccm. of decinormal acid corresponds to an excess of soda of 106 grammes per cubic metre, or, since 1 ccm. of decinor- 'mal acid is equivalent to .0061 of a gramme of Na SiO it corresponds to 122 grammes of Na SiO per cubic metre.

If, now, the degree of alkalinity of the water in the boiler is equal on an average to 7 .5 com. of decinormal HCl and if 2 ccm. are taken up by Na SiO and 5.5 ccm. by Na CO then 1 cubic metre of the water in the boiler contains 5.5 106==583 grammes of N21 CO and 24A grammes oi Na SiO Now, according to the analysis given above, the untreated feed water contains 10.7 mg. of SiO per litre.

Thereforethe purified water will contain 21.8 mg. of Na SiO per litre. If the maxieguivalent to .0053 of a gramme of soda or,

mum Na SiO content of the boiler water is not to exceed 244 grammes, then the boiler water must not be concentrated to more than times its original concentration.

Any desired degree of concentration can be easily estimated it any other salts present in considerable ipiantity are taken into consideration.

From the preceding calculations it will be seen, that the purified water contains 506 grammes of Na O, and 207 grammes of NaCl per cubic meter of water. If these salts are concentrated up to 11.2 times their original concentratiomthen 1 cubic metre of the water in the boiler will contain 506 X 11.2=approximately 5670 grammes of Na SO, and 207 11.2=approxiniately 2325 grannnes of NaCl. If the concentration of the solution in the boiler is measured in Baum degrees, then with an 11.2 fold concentration the Glaubers salt will give a figure oi ggg=approximately .044 B6.

and the common salt on the other hand will give a figure of The excess of soda present in the water amounting to 583 grammes per cublc metre gives a concentration of of the quantity of feed water is continuously drawn off from the boilers. In carrying this out it is not necessary to measure specially the amount of boiler water blown-off, inasmuch as an indication as to whether the right quantity has actually been withdrawn is always given by the measure ment of the concentration of the boiler water, so long as the prescribed degree of alkalinity is always approximately maintained.

The blowing-off of the boiler water is effected in such a manner that the heat is as far as possible recovered again. This can be attained by the provision of a thin pipe connected to a suitable point on the boiler and serving for conveying a certain quantity of the boiler water to an evaporator. In this evaporator the boiler water is placed under atmospheric pressure by means of a pipe. In consequence of this decrease of pressure the temperature is lowered so that heat is set free which causes a part of the boiler Water conveyed to the evaporator to evaporate. The steam generated in this Way is with drawn from the evaporator and conveyed into a water purifier or a feed Water container, in which the steam condenses. The quantity of steam formed in the container is dependent upon the original pressure, that is to say 011 the original temperature, of the boiler Water.

For example, at an absolute pressure of 16 atmospheres about 20% of the blown-off boiler water containing the salts is evaporated. The remaining water, the temperature of which has decreased to about 100 (1., in consequence of the decrease of pres sure, is led from the evaporator into a preheater in which it yields up the excess of its heat to the fresh feed water which is led in a counter current through the pre-heater.

In this Way about 90% of the heat present in the drawn-oft boiler Water is recovered,

A processfor preventing the formation of incrustation or scale in boilers using water containing an alkaline earth compound and salts ofsilicic acid, comprising blowing off the boiler and adding make-up water treated to provide an excess of sodium carbonate over sodium silicate in the water in the boiler, the amount of blow-0E, and the degree of alkalinity in and the amount of the make-up water being calculated to maintain the total concentration below a predetermined maximum.

Dated the 17th day of May, 1927.

In testimony whereof I have signed my name to this specification.

JULIUS OSTERTAG. 

